It is often desirable to obtain measurements of selected characteristics of sheet materials during manufacture. Although various properties of sheet materials can be detected by off-line laboratory testing, such tests often are not practical because of the time required for sample acquisition and analysis. Also, laboratory testing has the shortcoming that samples obtained for testing may not accurately represent sheet material that has been produced.
To overcome the drawbacks of laboratory testing of sheet materials, various sensor systems have been used for detecting sheet properties “on-line,” i.e., on a sheet-making machine while it is operating. Typically, on-line sensor devices are operated to periodically traverse, or “scan,” traveling webs of sheet material during manufacture. Scanning usually is done in the cross direction, i.e., in the direction perpendicular to the direction of sheet travel. Depending upon the sheet-making operation, cross-directional distances can range up to about ten meters or more.
A wide variety of scanning sensor devices has been developed for on-line measurements of sheet materials. As illustrated in FIGS. 4A and 4B, the scanning sensor system 100 typically includes a stationary frame 102, having a pair of upright end members that stand on a factory floor for supporting a guide member 104 that extends horizontally across a traveling sheet (not shown). A motor driven carriage is mounted to travel on the guide member 104. The carriage is connected to a drive system to be driven back and forth across sheet. The scanning sensor system also includes a scanning head 106 that is mounted on the carriage member. The scanning head 106 contains the detection components. For example, in the case of a spectrometric analyzer, the scanner head can include a source of infrared light, collimating and beam-splitting mirrors, and photosensitive detector.
The scanner head 106 is electronically connected by a large number of wires 108 that are connected to the process controller 110. The scanner head 106 travels back and forth along the cross direction adjacent the traveling sheet being analyzed. In FIG. 4A, the scanner head 106 is at one side of the frame 102 and is moving toward the middle of the frame 102 as shown in FIG. 4B. Wires 108 typically comprise a power chain or track that has either relatively flat or cylindrical, elongated structure that consists of a series of long, parallel conductors that are separated from one another by an insulating material. As the carriage moves back and forth, the wires are subject to fatigue as it undergoes cyclic motion, as well as by abrasion, impact or tension overload. In addition, the weight of the power chain causes the suspended wires 108 to sag because of the lack of adequate support. Consequently, the detection components, that are located in the scanner head, are subject to excessive vibrations. Moreover, the presence of the relatively heavy power chain makes it more difficult to drive the carriage and to control its speed. These phenomena combined reduce the reliability, efficiency and sensitivity of the detectors. The numerous wires in power chains are also subject to electromagnetic interference (EMI) and power losses. The art is in need of techniques for improving the operations of scanning systems by minimizing or eliminating the problems associated with power chains.